Rubber Milling: The Efficient Process of Transforming Raw Rubber into Everyday Products

Milling rubber is a necessary process in the manufacturing of rubber products. It is carried out in a controlled environment with different tools and techniques. All these aspects depend on the manufacturer and the use of the rubber. In this article, we cover these topics to help you get more insight into the rubber milling process.

What is Rubber Milling?

Whether natural or synthetic, rubber material must be thoroughly processed, tested, graded, and prepared for the manufacturing process. Essentially, these sets of activities are what define the rubber milling process.

Rubber milling is a technical process where raw rubber byproducts are transformed into usable rubber material. This process should not be confused with rubber mining which entails sourcing raw materials such as latex and converting it into rubber. These are industrial processes and are carried out by different companies.

The rubber that is produced from raw materials is then sold to rubber product manufacturers who then mill it accordingly. Still, there are manufacturers who have the capacity to handle raw material processing as well.

One of the characteristics of unprocessed rubber is that it comes in large chunks which are hard to take directly into the product manufacturing process. This rubber can also be impure, ungraded, and untested for properties.

Consequently, the rubber manufacturer has to take this unprocessed rubber and follow standard procedures to convert it into useful material. 

The rubber milling process is a crucial undertaking that helps rubber product dealers have the best quality products. Skipping this process and going directly into production will lead to poor-quality products that are not compliant with the regulations.

The Significance of Rubber Milling 

Transforming rubber into usable material is quite significant. It benefits you as the rubber products dealer, the manufacturer, and the environment at large. Let us explore these significances in more detail.

Processing Raw Materials for Rubber-based Products

As we said earlier, rubber in its natural form is not quite useful. It needs to undergo this vital process before it can be used. 

Most of the raw materials of rubber contain molecules and compounds that are harmful if uncontrolled. In the milling process, manufacturers remove or reduce these compounds from the material to make it safe. This is a benefit to you since it promotes the credibility of your brand. It is also beneficial since it controls the release of toxins to the general public.

Increases Workability of Rubber

Processing rubber makes it more flexible and usable. This means the milling process opens up possibilities for the material. For instance, when rubber is processed, it gets graded and categorized based on its properties. Subsequently, it allows the manufacturer to unlock more use cases for the material.

Workability is the ability to work with something efficiently. Unprocessed rubber is usually in large sheets or rolls. In this state, the material is not easy to work with. In the milling process, the sheets are cut into pellets and small marbles that are easy to load into a compressor or other machines.

Reach Industry Standards

The rubber industry is highly regulated and for good reasons. Internationally, the ASTM is responsible for stipulating rubber standards that manufacturers need to adhere to. These standards include test procedures, certifications, and other aspects.

Meeting these standards is critical for various reasons. One, it ensures that the final rubber products are certified for use. It also ensures that there is homogeneity in the industry. Rubber manufacturing companies strictly adhere to these standards for these reasons.

Attaining Consistency of Materials

When making rubber products, you want the materials to be of the same composition, grade, color, and so on. This is important since it ensures that the quality of products is maintained across the production line. Also, this contributes to products that have the right properties. In other words, consistency is key. 

Recycling Used Rubber

With global concerns in the manufacturing sector, recycling is deemed a progressive practice in the rubber industry. It helps handle used rubber products in the right way to avoid mismanaged waste. This is significant also since it contributes to the supply of raw materials for new products. In the end, it is a win-win situation for everyone.

Types of Rubber Used in the Milling Process

The term rubber is used to describe organic or inorganic substances that have elastic and waterproof qualities. That is the basic definition of rubber. Terming them as organic or inorganic means that they can either be made of natural or synthesized compounds. From these sources, we then categorized rubber as either natural or synthetic.

Natural Rubber

This is the organic form of rubber. Primarily, natural rubber is derived from latex, a white substance that exists in an ancient tree known as Hevea Brasiliensis. This tree is native to South America but it has been cultivated in many countries worldwide.

A chemical process known as coagulation is used to convert latex into rubber. In most cases, formic acid is used in this process. The latex is a dispersion. In other words, it is a solution of water and rubber particles. The coagulation process coagulates (solidifies) the rubber particles and forms a cohesive substance. When the process is over, the coagulated rubber is rolled using special machines and prepared for the next steps in the mining process.

Natural rubber has several properties. They include resistance to abrasion, tear, compression, and some substances. It is also relatively durable, resilient, and elastic. These properties make it reliable in many applications.

This type of rubber is classified based on the source of the latex. This means that there are other trees that produce latex apart from Hevea brasiliensis. They include Guayule, Russian Dandelion, Para Rubber Tree, and Jelutong Tree. Each of these latex sources produces rubber with different properties, characteristics, and use cases.

Synthetic Rubber  

With the help of chemical engineering, scientists have been able to successfully create substances that mimic natural rubber. These substances are collectively known as synthetic rubber. 

Synthetic rubber is made of a process known as polymerization. This involves the linking of monomer (single-chain molecule) agents into multi-chain molecules called polymers. It is a chemical reaction that should occur under controlled conditions for the outcome to be positive.

The properties and characteristics of this type of rubber are determined by the type of molecules used in the synthesis. Additionally, the types of molecules are used in the classification of the different types of synthetic rubber.

Mainly, there are three types of synthetic rubber;

  • Neoprene. A molecule known as chloroprene is polymerized to produce neoprene. This type of synthetic rubber is widely used across many industries for its excellent chemical stability and inertness. 
  • Nitrile rubber (Buna rubber). This type of rubber is derived from acrylonitrile (ACN) and butadiene compounds. Due to its resistance to oils and many chemicals, nitrile rubber is used in manufacturing components for automotive and aeronautical industries. 
  • Butyl rubber. Butyl is a copolymer (multi-polymer substance) that is manufactured from isobutylene with isoprene. It has superior resistance to gas permeability, which makes it an excellent material for manufacturing all sorts of parts that come into contact with gases.

The reference ‘rubber’ is used without limiting to either the synthetic or natural variation. It simply refers to all substances that have elastic properties. Therefore, this article will use rubber to mean either a natural or artificially synthesized elastic substance.

Rubber Milling Equipment 

In the rubber milling factory, there are different equipment used in the process. Each equipment has a specific purpose and it contributes to the final material. We are going to discuss these tools, focusing mainly on their purpose.

Rubber Shredding Machines 

While the milling process has several operations, the rubber needs to be first reduced into smaller sizes (rubber pellets) for easier working. This is where the milling machines come in handy. In general, there are two types of rubber milling machines. They are differentiated mainly by the approach they use to mill the rubber. Here are the two machines;

Hammer Mills

These are some of the common rubber shredding machines in the industry. Hammer mills have a rotating mechanism that rotates the hammers and cuts through the sheet of rubber that is fed into it. This whirling movement is quick to ensure the large sheets are cut precisely into small rubber granules.

Hammer mills serve a mechanical purpose. In other words, their sole purpose is to reduce the size of rubber and make it easy to transport. These machines are mostly used for shredding rubber right after it has been processed either by coagulation or polymerization.

Granulators

These milling machines are primarily used to shred used or recycled rubber. Depending on the size of the rubber product, the recycled material will vary in size. The granulator machines have a feeder section where the recycled material is loaded into the machine.

This machine works by shredding the rubber-based products into feedstock size to prepare them for reprocessing. Note that this is reprocessing and not processing. The difference is that recycled materials would need to undergo more processes before they are used again. It will involve cleaning, remelting, and so on.

Comparatively, granulators are cheaper and more accessible than hammer mills. This is primarily to promote more recycling of the rubber. Also, granulators are used in small-scale applications while hammer mills are for large-scale rubber millers.

Two-roll Rubber Mill

This machine is used in a process known as rubber compounding. In the first phase of processing rubber, technicians need to mix rubber compounds for an extended period. This creates a compound that has a consistent form. One of the most common machines used to achieve this is the two-roll rubber mill.

The two-roll rubber mill has two stainless steel cylindrical tubes that rotate in different orientations to achieve the rolling. Between the two rolls, there is a space where the rubber compounds are placed. The machine also has varying rolling speeds and this helps in its processing. 

The two-roll mill has a heating system that facilitates the mixing of the rubber compounds. In most cases, the cylindrical tubes supply the heat to the compounds. This heat is controlled to certain temperatures to ensure the properties of rubber are not destroyed.

Screens and Classifiers

Both these machines are used for the same purpose. When the rubber is shredded into granular sizes, it needs to be held before the next steps take place. Screeners and classifiers are used to hold the shredded rubber. 

Screeners and classifiers are technically not machines because they are not particularly mechanized like other machines. For one, the purpose they serve does not require mechanical interventions. If anything, they act like the receiving end of the whole process.

On top of being used to hold the processed rubber pellets, screeners and classifiers are also used when categorizing, grading, and packing rubber. This step is important since it helps manufacturers deal with the processed rubber more efficiently. 

Cooling Systems

In the rubber milling process, there are steps such as mastication which involves heating the rubber to quite high temperatures. This heat is essential in the mixing process since working with solid rubber is challenging.

After it has been heated, this rubber needs to be cooled down. On one hand, the cooling systems are there to ensure the temperatures are brought down. Also, this cooling helps maintain and preserve the properties of rubber.

Cooling systems can be independent or installed on other machines. For instance, the two-roll miller is equipped with a cooling system. This facilitates the subsequent heating and cooling from one station. Doing this saves time, space, and resources for the manufacturer. For you, that means that you can get competitive prices.

Dust Collection Systems

The rubber milling process produces a lot of waste in both liquid and solid states. Dust is a common example of waste products. Particularly, this is experienced when dealing with recycled rubber. For one, the use of rubber slowly reduces the structural integrity of the material. Therefore, a powdery substance is expected when scraping or grinding used rubber intensively.

Dust collection systems are installed in the milling stations to mitigate the negative effects of dust or rubber powder. When left uncontrolled, this dust could prove to be hazardous to people and the environment at large. These systems are also either integrated or independent. Most machines have tubes that are connected to a suction system leading to a disposal center.

Methods of Milling Rubber

There are several techniques used in the refinement of rubber. Over time, manufacturers have used one or the other methods depending on their applicability. This means that the method used depends on the manufacturer and their milling conditions. Let us discuss some of these methods.

  • Open milling – As the name suggests, this method is carried out in open setups. This method is used in small-scale applications where conditions are controllable. 
  • Closed milling – In these situations, the rubber compounds are completely restricted from getting contaminated by surrounding conditions. This method is essential when the goal is to produce the purest composition of rubber material through compounding.
  • Banbury Mixing – To achieve complex compounds of rubber, there are special machines required to handle the mixing requirements. For this, a Banbury mixer is a great example used for this process.
  • Cryogenic Milling – Cryogenic refers to the process of extremely lowering the temperature of substances. In the case of rubber, a cryogenic chamber is used to drop cool-down rubber to below-zero temperatures using liquid nitrogen. Cryogenic milling reduces the brittleness of rubber, making it easy to shred or grind into finer particles.
  • Wet Milling – This method of milling rubber involves the milling of rubber in the presence of a liquid substance. In most cases, water is used. However, there are other special solvents that can be used.

The methods of milling rubber are only limited to innovation. Each method has a specific objective that it aims to accomplish. Therefore, as the production process of rubber keeps evolving, newer and more effective methods will be introduced.

The Rubber Milling Process

For most conditions, the rubber milling process takes place in four steps. It is largely a systematic process which means that the steps need to be followed in a sequence. If done right, these steps lead to a material that has the right properties as expected. Let us explore each step in this process and what it entails.

Mastication

This is arguably the most time-consuming step in the milling process. Besides, it is the initial in a sequence of steps. By definition, mastication is the breakdown of the molecular components of rubber to prepare it for the compounding phase.

While mastication applies a physical approach, it affects the molecular structure of the rubber compound. Specifically, this process breaks down the polymer chains holding the material together. This decreases its brittleness and improves its elasticity. The implication here is that raw or unprocessed rubber is quite stiff and mastication reduces this stiffness.

Mastication is essential since it improves the elasticity of rubber, which is one of the most important properties of the material. 

Mastication is primarily aided by heat. Rubber is exposed to high temperatures until it melts. This heat is what breaks down the bonds of rubber. The two-roll mill main is the machine used in the mastication of rubber.

Mastication in itself has several sub-steps. During the heating process, rubber is mixed with compounds additives, and other substances. The goal of this is to enhance the innate properties of rubber and make it more versatile.

Mixing

In some situations, the mastication process does not include a mixing process. For instance, when the method of closed mixing is required, the mixing is separate and done independently.

Also, in cases where the two-roll mill is used for masticating rubber, the mixing action comes after the mastication has occurred. Mixing is essential since it facilitates the formation of consistent compounds of rubber.

There are several types of mixers that are used for this step. One of them is an Internal Mixer machine which is an enclosed machine containing rotors that knead the material. This mixer has an opening where the masticated rubber and additives can be fed. It also has a rotating mechanism that blends the rubber and the additives to achieve a homogeneous compound.

Shaping

After the rubber has been masticated and mixed properly, it needs to be shaped according to the applications that follow. Some products that do not require blending into other components are shaped in this step. A good example would be electrical wire insulators are processed in this step.

Methods such as injection and compression molding are used to shape rubber. The molds are what determines the shapes and designs of the products. For material that is not ready to form into products, it is formed into sheets or rods of rubber. These shapes are packaged and stored until the need arises.

Curing

This is the final step in the production of products using the rubber milling process. It is essentially a process where the once soft and flowy liquid rubber is hardened using curing agents. This is done while ensuring that the material retains its elastic properties.

Curing rubber is a relatively straightforward process. Mostly, it happens during the molding process. This is only if the products are being made using molding techniques. Therefore, curing is essential since it ensures that the rubber products are elastic but strong enough to resist tensile strength.

Heat is the most common rubber-curing agent. For this to happen, the mold is connected to a heat source when the material is loaded. This material is exposed to the heat for a measurable amount of time until the curing process is complete. Other curing agents of rubber include metal oxides, sulfur, and peroxides.

Quality Control in the Rubber Milling Process

Controlling the quality of milled rubber is a crucial task that all manufacturers perform. Quality control involves mainly tests and verification to ascertain that rubber has the properties that are expected. 

Testing Properties of Rubber 

The quality control tests of rubber are carried out before, during, and after the manufacturing process. The before tests are largely physical inspections to ensure that the material looks and feels as expected. Then for the ‘during tests’, engineers sample out some of the material and test for some properties. Finally, technicians perform the final tests to make sure the products exhibit similar properties as the rubber material.

Testing the properties of rubber is a standard practice across the industry. This means that the tests are designed by an independent regulator and not the manufacturer. The goal of this approach is to ensure objectivity and avoid bias in the setting of standards and procedures. Also, it unifies and creates a guideline for all manufacturers to follow.

Generally, the tests are broken down into mechanical and chemical steps. The chemical tests are to verify the chemical properties while the mechanical looks to ascertain the presence of the mechanical properties of rubber.

Some of the common quality control tests include;

  • Abrasion resistance test
  • Ozone resistance test
  • Cold resistance test
  • Shore (durometer) hardness test
  • Flex cracking test
  • Vertical Rebound resilience test

Each of the above tests is carried out separately at different phases in the manufacturing process of rubber and its products. The ASTM guides manufacturers on the certification requirements or the values needed for rubber to be regarded as safe for use.

Benefits of Quality Control 

One of the reasons why quality control is pushed by these organizations is for the benefit of the end-user of the product. Rubber is largely used in applications that require a lot of caution. As such, ensuring that the material has the right properties instills confidence. This translates to customer satisfaction which creates trust among businesses and customers.

Compliance is another benefit of quality control. As much as these regulators are concerned about the safety of the end user, they are also legally mandated to enforce compliance. This means that they have the authority to act against the manufacturers who fail to comply with the standards and procedures.

Finally, quality control leads to a more efficient rubber milling process. When you identify defects early on in the process, you can streamline the whole process. Moreover, a streamlined process is quite effective and this reflects on costs.

Properties of Natural Rubber 

We have been referring to the properties of rubber but we have not quite elaborated on them. The properties are simply the attributes that this material exhibits. More specifically, these properties describe the behavior of the material when exposed to certain conditions.

Natural rubber is known to handle stressors without degradation. Another thing is that the properties are used to estimate the lifespan of the material. This is why we are saying that natural rubber can handle thermal, chemical, and physical degrading elements.

Here is a list of the most common chemical and mechanical properties of natural rubber;

  • High tensile strength
  • Good abrasion resistance
  • High chemical resistance
  • Low electrical conductivity 
  • Moderate UV weatherable 
  • Low thermal conductivity 
  • Good adhesion
  • High elasticity 
  • Good insulator 

Evidently, this material is capable of handling many stressors. This makes it useful in applications with chemicals, heat, and tension.

Properties of Synthetic Rubber 

While the two materials are almost identical physically, they have varying properties. However, the differences are subtle. This is because synthetic rubber is a modified alternative to natural rubber and it is made to mimic natural rubber.

The main is that the properties of rubber can be precisely made since they are synthesized in the laboratory. One of the approaches used is to enhance and increase the limits of the values that are achieved by natural rubber. For instance, if the Elongation at the Break of natural rubber is 700%, the technicians can increase it in synthetic rubber using methods like reducing crosslinking.

One last thing to note is that the properties of synthetic rubber depend on the type. If you remember, we discussed three categories of synthesized rubber. Each of these materials has different compositions of compounds which affect the material’s properties. Therefore, properties like tensile strength, resistance to elements, and others vary. 

Application of Milled Rubber

Processed rubber is useful in almost all industries. In fact, there are over 10,000 products that are made of rubber. These products range from everyday items such as footwear to more niche products in industries like military equipment. The applications include;

  • Automotive parts. Mostly, rubber is used to make gaskets, sealing enclosures, insulators, and so on.
  • Aerospace seals and gaskets. The aerospace industry is among the major benefactors of rubber. Seals and gaskets made from rubber can sustain the thermal and chemical conditions expected in airplanes and other aerospace equipment. 
  • Home Accessories. In most of our homes, there are at least a dozen products that are made of rubber. Accessories like electrical appliances have rubber parts that support insulation and adhesion.
  • Medical devices. The medical industry also benefits a lot from the properties of rubber. For instance, medical gloves are made of rubber due to its resistance to chemicals.

Challenges in the Rubber Milling Process

The rubber milling process is not all smooth. There are challenges that are experienced throughout the process and they can affect the efficiency and quality of the final product. Some of these challenges include;

Handling Heat Transfer

In the mastication process of rubber, the material needs to be heated and cooled. The cooling is essentially the removal of the heat that was added to the material when heated. Handling the thermal balance here is a demanding endeavor.

If a mistake is made and rubber is overly heated, it can lose all its properties. Also, failing to cool it to the desired temperature will damage its molecular structure. Heat imbalances affect the mixing efficiency of rubber. This leads to inconsistent composition of compounds in the material.

High Energy Consumption 

Another caveat of rubber milling is the amount of energy consumed. The majority of the machines in the whole process need to be powered mainly by electricity. Energy consumption rises because some of the steps take as long as 12 hours. 

While this is virtually inevitable, energy consumption can have negative implications on finances, the environment, and safety. Therefore, most manufacturers try to be as efficient as possible in a bid to curb the energy consumption.

Differences in Raw Materials 

The raw materials of rubber have varying components. This is a challenge when milling rubber. One of the major challenges is recognizing the suitable techniques required when handling each material. This consideration is made to ensure that the properties of the material are preserved. In the end, this is all that matters.

Handling By Products

Rubber waste management has been a point of contention for decades. For one, rubber has poor biodegradability properties. Therefore, leaving it out in the open and expecting it to degrade naturally is not an efficient approach. Also, rubber is toxic and can affect the atmosphere if released without deliberation. 

Conclusion

Rubber milling is a short but complex process. Manufacturers need to be cautious to ensure the material has the right properties. The machines, tools, and methods used also have to be carefully chosen to ensure positive outcomes. Lastly, quality control is inseparable from this process for the sake of safety, value, and compliance.

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